1) Field of the Invention
The present invention relates to a high output semiconductor laser and a high output semiconductor laser module used for an optical fiber amplifier such as an EDFA (Erbium-Doped Fiber Amplifier) or a Raman amplifier.
2) Description of the Related Art
In recent years, the rapid spread of the Internet and the sharp increase in intra-company, inter-LAN connections have caused the problem of an increase in data traffic to arise. To solve this problem, a WDM (Wave-length Division Multiplexing) transmission system is developed and spread dramatically.
In the WDM transmission system, a plurality of optical signals are carried on different wavelengths, respectively, thereby realizing large capacity transmission that is 100 times as large as the conventional system through even a line of fiber. Particularly, the existing WDM transmission system requires an optical fiber amplifier such as an erbium-doped fiber amplifier (hereinafter “EDFA”) or a Raman amplifier. The use of such an optical fiber amplifier enables wide-band, long-distance transmission. The EDFA is an optical fiber amplifier that applies the principle that when a special fiber doped with an element of erbium is pumped by a pumping laser at a wavelength of 1480 nm or a wavelength of 980 nm, light in a band of a wavelength of 1550 nm, serving as a transmission signal, is amplified in the special fiber.
The Raman amplifier is a distribution type optical fiber amplifier that employs an ordinary transmission-path fiber as a gain medium without the need of the special fiber such as the erbium-doped fiber, unlike the EDFA. The Raman amplifier can realize a transmission band that has a flat gain in a wider band as compared to that of the conventional EDFA-based WDM transmission system.
Therefore, in order to realize an improvement in the reliability of the WDM transmissions system and a decrease in the number of relays thereof, a high output semiconductor pumping laser that operates stably in a single transverse mode is necessary for the optical fiber amplifier. As this pumping laser, a buried hetero-type semiconductor laser device (BH laser) is effective. The buried hetero-type semiconductor laser device includes an active layer having a quantum well structure, more preferably a multi-quantum well structure (MQW structure) in which the active layer consists of a plurality of quantum wells and baffler layers. Actually, a semiconductor laser module having such a semiconductor laser device packaged therein is employed as the pumping light source for the optical fiber amplifier.
There is known, as a technique for realizing the high output of the semiconductor laser device, one for forming an active layer to have a multi-quantum well (MQW) structure, particularly a strained MQW structure. The MQW structure is realized by forming heterojunctions between well layers and barrier layers alternately arranged and made of semiconductor materials. At each of the heterojunctions in particular, the barrier layer has wider bandgap energy than that of the well layer. Further, it is known that the strained MQW structure is a structure in which a semiconductor material of the well layer and a semiconductor substrate are different from each other in lattice constants, and that the strained MQW structure enables further improvement in performance of the semiconductor laser device.
The semiconductor laser device that has the active layer of the MQW structure often uses a separate confinement heterostructure (SCH) that functions as an optical waveguide in each of lower and upper portions adjacent to the active layer of the MQW structure. By using the SCH, it is possible to confine a laser beam generated in the active layer more efficiently, and to realize high output operation.
There is known a technique for enabling higher output of the laser by using a GRIN-SCH (Graded-Index-Separate Confinement Heterostructure) structure in the semiconductor laser device.
Meanwhile, it is known to adopt a long resonator structure as a unit for realizing far higher optical output in the semiconductor laser device. Making a resonator longer decreases both electric resistance and thermal impedance of the device, thus improving optical output saturation (heat saturation) caused by heat generation. This means an increase in not only the maximum optical output but also a driving current (saturation diving current) for saturation output (maximum optical output). Eventually, the semiconductor laser device that employs the long resonator can realize high output operation with low power consumption when the device is driven with a large current, as compared with the conventional semiconductor laser device. However, the semiconductor laser device that employs the long resonator has the following disadvantage. When a driving current is restricted to a certain degree, the influence of internal loss becomes dominantly larger as the resonator is longer. This results in the deterioration of external differential quantum efficiency and the lowering of optical output.
As another means for realizing high optical output, it is known to increase the width of the active layer of the semiconductor laser device. By widening the width of the active layer, it is possible to decrease both the electric resistance and thermal impedance of the semiconductor laser device, and to enable high output with a large current.
Therefore, it is the key to realizing higher output by increasing the width of the active layer as much as possible in the semiconductor laser device having the GRTN-SCH-MQW structure that includes a relatively long resonator. However, the active layer that includes the SCH also acts as an optical waveguide. Therefore, increase in the width of the active layer stimulates occurrence of a higher-order transverse mode. When the driving current of the laser increases a kink such that optical output becomes discontinuous at a certain driving current occurs more easily. In order to improve the stable operation of the semiconductor laser device and the manufacturing yield thereof, the occurrence of a kink should be avoided. Japanese Patent Application Laid-Open No. 8-330671, for example, discloses that the width of the active layer is required to be smaller than 1.8 μm so as to avoid the occurrence of the kink.